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Line profile creation with RSpec, VSpec and Fitswork

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#1 mwr

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Posted 12 December 2019 - 05:04 PM

For excitation class determination of planetary nebula intensities of [O III] and H beta lines need to be measured. Line profiles from slitless spectra can be created by RSpec, VSpec and Fitswork. I have compared the created line profiles from a spectrum of the planetary nebula IC 2419 that was recorded using the Star Analyser 200 and  a Canon 450 Da. The outcome is quite surprising because I have obtained very different results for the line intensities:

 

RSpec_vs_VSpec.jpg

 

RSpec creates a significantly weaker H alpha line at 6563 A than RSpec. Obviously the algorithms for RGB to luminance conversion are quite different. Fitswork delivers yet another profile:

 

fitswork.jpg

 

The published relative line intensities (Astrophys. J. 426:653, 1994) are:

 

H beta :        100

[O III] 4959:  184

[O III] 5007:  565

H alpha:       312

 

The best match is obtained from the line profile created by Fitswork. Are there settings in VSpec or RSpec that I'm not aware of and that allow to adjust the RGB to luminance conversion? 

 

 

 



#2 robin_astro

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Posted 12 December 2019 - 08:50 PM

Have you subtracted the background and corrected for the instrument response?  (With a planetary nebula the continuum should be almost zero) The instrument response may be different depending on how you converted RGB to mono but as long as you use the same conversion for calculating the instrument response and the target you should get the right result. (Note you should always use raw output from DSLR as other formats can apply white balance corrections which will change the instrument response)  

 

How have you measured the line strengths?  (Since the lines will be much narrower than the resolution, the height will not be a good measure so you have to measure the flux by measuring the area of the line. With absorption lines you  do this by measuring the equivalent width (effectively the fraction of absorption relative to the continuum)  but for emission lines you measure the area 

 

Robin



#3 robin_astro

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Posted 13 December 2019 - 03:55 AM

In general it can be difficult to get good quantitative flux values with colour cameras because you may not have good even coverage of the different colour pixels, particularly R and B



#4 mwr

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Posted 13 December 2019 - 03:56 PM

 (Note you should always use raw output from DSLR as other formats can apply white balance corrections which will change the instrument response)  

 

 

Hello Robin,

 

your hint of verifying the raw output was a good one. I have just opened one Canon CR2-raw file with RSpec and have created the line profile (SA-100 spectrum). The blue/green [O III] and H beta lines are well detected but the red H alpha line is barely visible although a red dot is clearly present in the raw spectrum:

 

RSpec_raw.jpg

 

 

Obviously I have a problem with the RGB to luminance conversion in RSpec already at the level of the raw file. Using VSpec the line profile is correctly created. Any suggestions how to fix this problem in RSpec?



#5 robin_astro

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Posted 13 December 2019 - 06:27 PM

 

Obviously I have a problem with the RGB to luminance conversion in RSpec already at the level of the raw file. Using VSpec the line profile is correctly created. Any suggestions how to fix this problem in RSpec?

I can't help with this one, I am afraid as I don't use RSpec. It is a while since I used colour cameras too. I have used VSpec but did not realise it accepts raw colour files. This is the technique I followed when I used a DSLR, using IRIS (now ISIS) to convert the raw files to mono fits

http://www.threehill...roscopy_11a.htm

 

RSpec has its own group which might be able to offer advice

https://groups.io/g/RSpec-Astronomy

 

 

Cheers

Robin


Edited by robin_astro, 13 December 2019 - 06:27 PM.

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#6 mwr

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Posted 13 December 2019 - 10:45 PM

Thanks Robin!

 

 I have used VSpec but did not realise it accepts raw colour files. 

 

For using VSpec I had to convert the raw files first into bmp files by Fitswork. 



#7 mwr

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Posted 14 December 2019 - 10:06 AM

 

How have you measured the line strengths?  (Since the lines will be much narrower than the resolution, the height will not be a good measure so you have to measure the flux by measuring the area of the line. With absorption lines you  do this by measuring the equivalent width (effectively the fraction of absorption relative to the continuum)  but for emission lines you measure the area 

 

Finally, I could circumvent the RGB conversion problems in RSpec by doing this conversion in Fitswork and then did re-open the converted file in RSpec. 

 

IC2946_label.jpg

 

The line strenghts (equivalent width) were measured and I dared to calculate the Balmer decrement and the excitation class (the absolute error was estimated by adopting a relative standard deviation for the equivalent width measurement of 10% and applying the concept of error propagation):

 

Balmer decrement: 5.0 +/- 0.8 (reported value for IC 2149 based on emmsission line fluxes: 3.8)

Excitation class: (5007+4959)/H beta: 9.11 +/- 1.4 --> low excitation class 2 (reported value: 7.7 --> low excitation class 2)

 

The Balmer decrement is quite high because H alpha and [N II] at 6583 could not be resolved. However, a rough determination of the excitation class was possible with the SA-200 setup.

 

I will try to determine the excitation class of other quasi stellar planetary nebula of medium and high excitation to verify if this can be achieved with  reasonable results by low resoltuion slitless spectroscopy:

 

IC 1747 Cas (8,7 magnitudes per sqr arcsec)

IC 351 Per  (7.3 m)

IC 2003 Per (7.4 m)

 

(more suggestions for appropriate targets under the western european sky are highly welcome)

 

Another excellent result for IC 418 in Lepus  using the SA-100 has been recently posted here: https://www.cloudyni...copy/?p=9789039 and it would be great to see some other examples.

 

 

 

 

 

 

 

 



#8 robin_astro

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Posted 14 December 2019 - 11:26 AM

Take care when measuring emission line strengths using Equivalent Width.  EW measures the line strength relative to the local continuum so if that is different for the different lines you don't get a direct comparison of the flux in each line. For emission lines you need to measure the flux directly in an instrument response corrected spectrum, independent of the continuum ie the area in the line with the continuum subtracted (not divided out as would be done for EW measurements)

 

Cheers

Robin



#9 robin_astro

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Posted 14 December 2019 - 12:24 PM

Take care when measuring emission line strengths using Equivalent Width.

As an example, consider this spectrum of Gamma Cas by Hugh Allen taken from the BAA database.

 

gamcas_20170827_915_Hugh Allen.png

 

The purple line is the original spectrum calibrated in relative flux (ie corrected for instrument response and atmospheric extinction)

The blue line is the rectified spectrum ie divided by the continuum as would be done to measure Equivalent Width)

The black line shows the original spectrum with the continuum subtracted. ie showing the flux of the emission lines at their correct relative intensities.  

 

Note how the ratio of the H alpha/H beta line strength is quite different in each case (EW measurement from the rectified spectrum gives an incorrect ratio of 8.2 compared with the correct ratio of 3.1 from the continuum subtracted spectrum.

 

Cheers

Robin



#10 mwr

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Posted 14 December 2019 - 01:23 PM

Take care when measuring emission line strengths using Equivalent Width.  EW measures the line strength relative to the local continuum so if that is different for the different lines you don't get a direct comparison of the flux in each line. For emission lines you need to measure the flux directly in an instrument response corrected spectrum, independent of the continuum ie the area in the line with the continuum subtracted (not divided out as would be done for EW measurements)

 

Cheers

Robin

Thanks Robin. VERY helpful! This pitfall can be easily overseen. In Richard Walker's  paper it is not clearly distinguished between the EW of absorption and emmission lines when relative intensitiy measurements at different wavelenghts are required:

 

"Analysis and Interpretation of Astronomical Spectra - Theoretical Background and  Practical Applications for  Amateur Astronomers"

https://www.unitroni...onomical-sp.pdf

 

In chapter 7 he states:

 

"EW values of absorption lines are by definition always positive (+), those of emission lines negative (-). Since the EW   value is always measured at a continuum level, normalized to Ic=1, it is neither influenced by the course of the continuum, nor by the absolute radiation flux. Should  EW be measured in a non rectified profile, the continuum must be normalised immediately at the base of the spectral line to Ic=1!"


Edited by mwr, 14 December 2019 - 01:25 PM.


#11 robin_astro

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Posted 14 December 2019 - 03:57 PM

What Richard Walker says is correct. The problem lies in the use of EW. By definition EW is only a measurement of line strength relative to the continuum. 

This is very useful for absorption lines where the interesting information is what fraction of the available light is being absorbed, but much less useful for emission lines which are generally formed independent of the continuum and only have a loose physical connection with it (In the case of gamma Cas for example it comes from a circumstellar disc, excited by light from the parent star in a totally different wavelength range in the UV)   

EW is popular as it is very easy to measure since does not need any flux calibration (You can get the correct result on raw spectra without any calibration other than for wavelength)  but its application to emission lines is limited, except perhaps where you just want to monitor changes in a particular emission line. Even then it only works provided you know the intensity of underlying continuum is constant at the wavelength being measured. 

 

Cheers

Robin


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#12 robin_astro

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Posted 14 December 2019 - 04:27 PM

Even then it only works provided you know the intensity of underlying continuum is constant at the wavelength being measured. 

 

Cheers

Robin

A good example of this is the strength of H alpha emission in novae as they evolve, as I calculated here for nova Del 2013 for example

 

Novadel2013_Halpha_flux.png

 

Just looking at the spectra and as measured using EW (pink line) it appears that H alpha line grows continuously but that neglects the fact that after maximum brightness the nova is fading  and cooling ie the continuum is getting weaker and redder.  Once corrected for this to produce the true line strength it becomes clear that the H alpha emission did not continue climbing but leveled out (blue line)

 

Cheers

Robin


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#13 mwr

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Posted 14 December 2019 - 04:28 PM

Have you subtracted the background and corrected for the instrument response?  (With a planetary nebula the continuum should be almost zero) 

 

How have you measured the line strengths?  (Since the lines will be much narrower than the resolution, the height will not be a good measure so you have to measure the flux by measuring the area of the line. With absorption lines you  do this by measuring the equivalent width (effectively the fraction of absorption relative to the continuum)  but for emission lines you measure the area 

 

Robin

After solving the problems concerning the RGB conversion in RSpec and after having received  very helpful advice from Robin here is the workflow for excitation class determination of IC 2149 using a DSLR and the SA-200:

 

1. Stacking of 10x 300 sec subframes in Fitswork

2. RGB to luminance conversion in Fitswork

3. Line profile creation in RSpec and wavelength calibration:

 

IC2946_raw.jpg

 

4. Background substraction in RSpec:

 

IC2946_raw_bckg.jpg

 

5. Correction for instrument response (blue curve) in RSpec:

 

IC2946_bkg_response.jpg

 

6. Area measurement of the emmission lines in VSpec (RSpec does not offer this function):

 

IC2946_bkg_response_area.jpg

 

Calculation of excitation class: area  [O III] lines / area H beta = 7.1 ---> class 2 (reported value 7.7)

Calculation of Balmer decrement: area H alpha / H beta = 5.4 ; H alpha and [N II] not resolved ! (reported value 3.8)

 

Hopefully I haven't forgoten important points. Comments are welcome.



#14 mwr

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Posted 14 December 2019 - 05:01 PM

A good example of this is the strength of H alpha emission in novae as they evolve, as I calculated here for nova Del 2013 for example

 

attachicon.gifNovadel2013_Halpha_flux.png

 

Just looking at the spectra and as measured using EW (pink line) it appears that H alpha line grows continuously but that neglects the fact that after maximum brightness the nova is fading  and cooling ie the continuum is getting weaker and redder.  Once corrected for this to produce the true line strength it becomes clear that the H alpha emission did not continue climbing but leveled out (blue line)

 

Cheers

Robin

Excellent example and explanation! With your incredibly rich experience in practical spectroscopy you should consider to write a book on this topic.



#15 robin_astro

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Posted 15 December 2019 - 08:14 AM

 

Calculation of Balmer decrement: area H alpha / H beta = 5.4 ; H alpha and [N II] not resolved ! (reported value 3.8)

 

 

Yes the [NII] contamination will mean the decrement is over estimated.  I went searching for a spectrum of IC2149 (is IC2419 a typo?) and found this website

https://web.williams...earch/index.php

The [NII] line strengths are quite small compared with H alpha but their removal would definitely move your result in the right direction

 

Cheers

Robin


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#16 mwr

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Posted 15 December 2019 - 09:25 AM

Yes the [NII] contamination will mean the decrement is over estimated.  I went searching for a spectrum of IC2149 (is IC2419 a typo?) and found this website

https://web.williams...earch/index.php

The [NII] line strengths are quite small compared with H alpha but their removal would definitely move your result in the right direction

 

Cheers

Robin

Thanks! It's indeed IC 2149. 

 

I have reprocessed the raw data using IRIS instead of Fitswork / RSpec for RGB conversion and sky background subtraction. The results are significantly different for the H alpha line strenght (blue curve: IRIS/Vspec; red curve: Fitswork/RSpec/VSpec):

 

aur_iris_vspec_vs_rspec_fitswork.jpg

 

Obviously the RGB conversion algorithms are not the same in IRIS and Fitswork (maybe there is also a significant difference in the sky substraction processing)! Luminance conversion is apparently a difficult task: http://openaccess.th..._2017_paper.pdf

 

Recalculation of the values for excitation class and Balmer decrement yields:

 

excitation class (oxygen/hydrogen line ratio): 7.8 (published value: 7.7)

Balmer decrement: 4.5 (4.1 when corrected for [N II]; published value: 3.8)

 

I'm starting to understand why most of the experienced ProAms are using IRIS and VSpec and CCD cameras .......

 

Are there good books on the market on applied astronomical spectrum processing? 


Edited by mwr, 15 December 2019 - 10:46 AM.


#17 robin_astro

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Posted 15 December 2019 - 10:53 AM

Interesting. A mono camera is definitely the way to go for quantitative spectroscopy. The two methods should still give the same result though provided your instrument response is calculated using the same method. (The golden rule is to process the target and reference in exactly the same way) 

 

I have just been reading Francois Cochard's "Successfully Starting in Astronomical Spectroscopy".

https://www.amazon.c...l/dp/2759820262

 

It is a very good practical book aimed at amateurs wanting to produce research quality spectra and is written by someone who has actually done this ;-)  It is based around slit spectrographs and mono cameras and uses ISIS as example software and does not go into the spectrum analysis side either so probably what not what you are looking for at the moment.


Edited by robin_astro, 15 December 2019 - 10:53 AM.

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#18 mwr

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Posted 15 December 2019 - 12:14 PM

(The golden rule is to process the target and reference in exactly the same way) 

 

What would be a good reference object then for determining the instrument response if the target is a planetary nebula? I have determined the response of my camera using a standard B star (red curve below vs. a bue curve that has been measured in the lab for my Canon 450 Da). I have learned from Walker's paper on spectra analysis that this method does actually not deliver the correct instrumental response curve. Different spectral classes generate different correction curves. I really need to get the book that you recommended so I don't need to bother you all the time ;-)

 

canon_450Da_response_curve.jpg

 


Edited by mwr, 15 December 2019 - 12:22 PM.


#19 Organic Astrochemist

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Posted 15 December 2019 - 01:16 PM

Thanks mwr for another excellent thread. I really think you should be applauded for showing what can be done with rather modest equipment.

Robin mentions the golden rule of processing the target and the reference the same way. When using my ALPY I take a spectrum of a nearby standard early type star (similar airmass) and create an instrument response curve so that the reference spectrum very closely matches the spectrum in Miles or Pickles database. Then I use this curve to process my target. I could do something similar with my star analyzer, but I’m lazy so I just use one of about three instrument response curves that gives me the best results. I’m curious what you do.

I also noted that not unexpectedly you seemed to use longer integration time with the SA200. Now that you have both, will you still use both the SA100 and SA200?

The forecast for tonight is good; maybe I’ll get to look at IC 2149.

Edited by Organic Astrochemist, 15 December 2019 - 01:19 PM.

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#20 Organic Astrochemist

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Posted 15 December 2019 - 01:30 PM

Interesting. A mono camera is definitely the way to go for quantitative spectroscopy. The two methods should still give the same result though provided your instrument response is calculated using the same method. (The golden rule is to process the target and reference in exactly the same way)

I have just been reading Francois Cochard's "Successfully Starting in Astronomical Spectroscopy".
https://www.amazon.c...l/dp/2759820262

It is a very good practical book aimed at amateurs wanting to produce research quality spectra and is written by someone who has actually done this ;-) It is based around slit spectrographs and mono cameras and uses ISIS as example software and does not go into the spectrum analysis side either so probably what not what you are looking for at the moment.


Robin can you give examples of ProAm publications based on star analyzer results?

Also, just to make sure I’m following, with respect to EW and dividing vs subtracting the continuum. In the case of planetary nebulae where the continuum level is often very low, correction for instrument response is necessary but correction for the continuum is probably moot, right?

#21 robin_astro

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Posted 15 December 2019 - 01:32 PM

 In Richard Walker's  paper it is not clearly distinguished between the EW of absorption and emmission lines when relative intensitiy measurements at different wavelenghts are required:

 

"Analysis and Interpretation of Astronomical Spectra - Theoretical Background and  Practical Applications for  Amateur Astronomers"

https://www.unitroni...onomical-sp.pdf

 

 

That is an early (bootleg) copy of his book which was finally published as "Spectroscopy for Amateur Astronomers"

https://www.amazon.c...n/dp/1107166187

 

You can see in the early versions that he had not got this issue straight in his own mind then. The published copy has this correctly, though he makes rather a meal of it and still does not explain how the practical measurement can be made. See attached.

 

Balmer_Decrement.jpg

 

This could be a useful book for you but it does seem to take a rather pessimistic view of what amateurs can measure and I have not gone through it in detail.

 

Cheers

Robin


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#22 robin_astro

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Posted 15 December 2019 - 01:48 PM

What would be a good reference object then for determining the instrument response if the target is a planetary nebula? I have determined the response of my camera using a standard B star (red curve below vs. a bue curve that has been measured in the lab for my Canon 450 Da). I have learned from Walker's paper on spectra analysis that this method does actually not deliver the correct instrumental response curve. Different spectral classes generate different correction curves. I really need to get the book that you recommended so I don't need to bother you all the time ;-)

 

You can use the usual hot star at similar elevation as normal. Walker is incorrect in stating different spectral classes produce different instrument responses.(Hopefully he does not say this in his final published version, though I have not checked)

I remember having this debate with him when he first started writing the on line version of his book. I even went to the trouble of demonstrating it for the Star Analyser and then again for the ALPY (twice!) It is an excellent exercise to do to prove to yourself that you can take good quality spectra which agree with what the professionals get. You can see the results of the tests I did here.

http://www.threehill...troscopy_21.htm

 

With good technique, the instrument response is of course the same for any object being measured (It is just as well it is, otherwise it would be impossible to measure the spectrum of an unknown object !)

 

Cheers

Robin


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#23 robin_astro

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Posted 15 December 2019 - 02:05 PM

Robin can you give examples of ProAm publications based on star analyzer results?

Also, just to make sure I’m following, with respect to EW and dividing vs subtracting the continuum. In the case of planetary nebulae where the continuum level is often very low, correction for instrument response is necessary but correction for the continuum is probably moot, right?

provided you have corrected for instrument and atmosphere and the continuum is effectively zero you can just measure the line strength directly. (not EW of course, that will give you a crazy answer where there is no continuum to measure relative to)

 

The main use in the pro field for the Star Analyser has been to give an early alert of the nature of new objects rather than anything quantitative. There are a couple of peer reviewed papers which mention the Star Analyser I believe, one is discovery of a new WR star and another is using it to measure high speed variations in spectra stochastically looking for fast instabilities in stars. I will try to dig them out. There are  a few novae that have been first recorded using a Star Analyser and possibly mentioned in ATels.  There have also been a few poster papers presented at pro conferences, one on Nova Del 2013 I think. This spectrum I took was the first to suggest an increase in brightness could be a very rare near field gravitational lensing event

http://www.threehill.../spectra_30.htm

Another interesting application is looking for rapid short lived outbursts in T Tauri or flare stars

I detected an example here

http://www.threehill.../spectra_42.htm

and Andrew Smith is commisioning an automatic monitoring system using a similar technique here

https://stargazerslo...24-first-flare/

Italian Amateur Claudio Balcon has built a slit spectrograph using an SA100, similar to my ALPY 200 and has confirmed and classified a few supernovae officially.

https://wis-tns.weiz...=Claudio Balcon

 

This is no means an exhaustive list and if anyone knows other applications for the Star Analyser in the pro field I would love to hear about them

 

Cheers

Robin

 

Cheers

Robin


Edited by robin_astro, 15 December 2019 - 02:09 PM.

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#24 mwr

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Posted 15 December 2019 - 03:17 PM

 I could do something similar with my star analyzer, but I’m lazy so I just use one of about three instrument response curves that gives me the best results. I’m curious what you do.

I also noted that not unexpectedly you seemed to use longer integration time with the SA200. Now that you have both, will you still use both the SA100 and SA200?

The forecast for tonight is good; maybe I’ll get to look at IC 2149.

So far I have mainly done qualitative analysis of SA-100 spectra and did only rectifiy some of them (I'm even more lazy than you ;-). Your post on IC 418 did inspire me to work on the measurement of excitation classes of planetary nebula and this forced me to do the SA-200 upgrade and to apply instrument response curves to get meaningful results. As you can see in this thread quantitative analysis of spectra is much more demanding for a beginner and Robin's clarifications and advice were very helpful. 

 

I will still use the SA-100 on dim objects (IC 2003 and IC 351) - every photon counts!

 

Good luck with IC 2149. I'm curious to see the results with your grab and go setup!


Edited by mwr, 15 December 2019 - 03:19 PM.


#25 mwr

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Posted 22 December 2019 - 11:10 AM

I also noted that not unexpectedly you seemed to use longer integration time with the SA200. Now that you have both, will you still use both the SA100 and SA200?

 

I have tested the SA-200 setup (f/9.4) on a dim stellar planetary nebula (IC 2003; mv = 12.60 according to CdC). The point spread function shows indeed an almost Gaussian star-like profile of this nebula:

 

Folie1.JPG

 

To measure at least H alpha/beta and [O III] lines with a reasonable signal to noise ratio 7 min. (!) subframes had to be taken (25 frames were stacked). The diagnostic important He II line for excitation class determination could not be detected:

 

Folie2.JPG

 

For objects showing emmission line spectra  the limiting magnitude of my system is estimated to be around 12 mag. A comparison of the spectra of IC 2003 (high excitation class; blue curve) and IC 2149 (low excitation class; red curve) shows that at least a semi-quantitative analysis is possible (both spectra were background and instrument response corrected):

 

Folie3.JPG

 

The [O III] lines in IC 2003 are more intense with respect to the H alpha and beta lines than in IC 2149 which is in accordance with literature data. 

 

To increase sensitivity with respect to excitation class determination of planetary nebula the substitution of my Canon 450 Da by a CCD camera will be certainly helpful. Recommendations are welcome! Any experiences with the Orion StarShoot G3 - monochrome CCD Kamera?


Edited by mwr, 22 December 2019 - 02:55 PM.

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